This invention relates to improvements in gas separation devices, particularly hydrogen diffusion membranes.
Hydrogen is one of the most important industrial gases. It is used, for example, in ammonia synthesis, methanol synthesis, fuel cells, chemical hydrogenations, gas chromatography, semiconductor processing, metal manufacture, glass processing and also as a cooling medium in power stations. In most of these applications, the hydrogen has to be virtually 100% pure.
In recent years, synthetic permeable membranes have been developed which can be used for hydrogen separation and purification. A purification technique which is based on the selective diffusion of hydrogen through bundles of fine silver/palladium alloy tubes has been employed for some years. However, this technique has not been universally accepted as a gas clean-up device due to its extremely high cost, high operating pressure and relatively low throughput. Improvements have been made in recent years where a silver/palladium alloy is deposited on a ceramic substrate. This allows layers of metal to be made much thinner, thereby reducing cost and giving a higher specific throughput at a given operating pressure. Other membranes which have been developed for hydrogen separation and purification include ceramic membranes, zeolite membranes and polymer membranes.
Palladium-based membranes are useful for a number of industrial and analytical applications. One such application is the processing of reformate gas streams to produce pure hydrogen for use in fuel cells. This hydrogen purification process has the advantage of being a single stage process which is compatible with reformate gas streams and operates at temperatures and pressures coincident with the normal reforming/cracking conditions (ie methanol, methane and other hydrocarbons). The palladium alloy may be deposited on the porous support at desirable thicknesses using a variety of methods, of which sputtering, chemical vapour deposition, physical vapour deposition and electroless plating are examples.
Although supported palladium alloy membranes of the above type, with very high specific flow rates, have been manufactured with some degree of success, the coating of a porous ceramic support with an essentially non-porous thin alloy film represents special problems. Small defects in the support lead to pin-holes in the palladium alloy membrane which compromise the maximum hydrogen purity which such composites can attain. Furthermore, the very important application of hydrogen processing for fuel cells requires hydrogen with a very low carbon monoxide content (typically less than 100 ppm for low-temperature phosphoric acid fuel cells and less than 10 ppm for proton exchange membrane fuel cells). This gas quality is close to the current state of the art for supported palladium alloy membranes and for such a critical application an additional degree of security is required. There is currently a high failure rate in the production of totally leak-free supported palladium alloy membranes and if a leak develops during use of such a membrane in a fuel cell system, the increased level of carbon monoxide can have an immediate poisoning effect on the fuel cell anode catalyst.
European Patent No 0434562 B1 relates to a process and apparatus for the purification of hydrogen gas streams used for hydrogenations in refinery and petrochemical plants. The carbon monoxide in such purified hydrogen gas streams needs to be less than 50 ppm. In this purification process, the hydrogen stream to be purified is firstly applied to a gas diffusion membrane capable of preferentially allowing hydrogen to pass through and at the same time preferentially blocking other components of the gas stream such as carbon monoxide and hydrogen sulphide. Most of the carbon monoxide in the original hydrogen stream is removed by the membrane but a small amount of carbon monoxide passes through the membrane. The permeate gas stream is then subjected to a subsequent and separate stage of methanation wherein the carbon monoxide content is lowered to the required level. The process and apparatus described in the aforementioned European Patent is intended for large scale industrial operation at high pressures and high flow rates. Feed gas pressures of 40 to 120 bars, pressure drops of 30 to 80 bars and flow rates of 12,700 Nm3/hr (ie over 200,000 liters/min) are mentioned. Moreover, because of the large volume of hydrogen involved, the two-step purification process (gas diffusion and methanation) is suitably conducted in several stages. Also, the gas diffusion and methanation steps are conducted at different temperatures.
The present invention provides an improved process and apparatus for the purification of hydrogen gas streams by a combination of gas diffusion membrane and methanation.
The present invention also provides a gas separation device which overcomes the problems of current gas diffusion membranes by preventing leakage of carbon oxides through the membrane.
According to the present invention there is provided a gas separation device in the form of a composite comprising a hydrogen diffusion membrane and a methanation catalyst for the removal of carbon oxides from hydrogen gas streams.
Suitably, the hydrogen diffusion membrane is associated with the upstream surface of a porous or microporous support and the methanation catalyst is associated with the downstream surface of the porous or microporous support.
Preferred support materials include alumina and alumino silicates.
Suitable hydrogen diffusion membranes include palladium alloy membranes, ceramic membranes, zeolite membranes and polymer membranes. Examples of ceramic membranes are a porous glass membrane marketed under the trade mark xe2x80x9cVycorxe2x80x9d which does not require a ceramic support and a metal oxide membrane marketed under the trade mark xe2x80x9cVelteropxe2x80x9d. Examples of polymer membranes are polyimides and polysulfone membranes marketed under the trade mark xe2x80x9cPrismxe2x80x9d.
Preferred palladium alloy membranes are palladium alloyed with one or more metals selected from Ag, Au, Pt, Cu, B, In, Pb, Sn and rare earths.
The palladium alloy membrane is preferably from 1 to 10 microns thick.
In the gas separation device of the present invention the methanation catalyst preferably is a selective methanation catalyst for the removal of carbon monoxide and/or carbon dioxide from a hydrogen gas stream.
Suitable methanation catalysts are those based on iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium or platinum.
In the case of carbon monoxide removal from reformate gas mixtures, particularly for fuel cell applications, the methanation catalyst suitably should be capable of reducing carbon monoxide in the permeate gas stream to a concentration below 100 ppm, preferably below 10 ppm.
A further aspect of the invention provides a process for the purification of a hydrogen gas stream using the gas separation device as claimed herein.
Suitably, the hydrogen gas stream to be purified is a reformate gas mixture.
Preferably, the hydrogen gas stream is fed to the gas separation device at a pressure less than 30 atmospheres.
Further preferably, the pressure drop of the hydrogen gas stream over the gas separation device is less than 15 atmospheres.
Suitably, the flow rate of the hydrogen gas stream fed to the gas separation device is less than 10,000 liter/min.
Suitably, the hydrogen diffusion membrane and the methanation catalyst function within the same temperature window.
Suitably, also the hydrogen gas stream is purified in a single pass through the gas separation device.
The present invention is also a fuel cell system for vehicular application comprising (a) an on-board hydrogen supply unit; (b) a hydrogen purification unit and (c) a fuel cell wherein the hydrogen purification unit comprises a gas separation device as claimed herein and operates by the process claimed herein.
Suitably, the fuel cell is a proton exchange membrane fuel cell or a low-temperature phosphoric acid fuel cell.
Embodiments of the invention will now be described by way of example only.